It's easy enough, I think, if you have your pages as bitmaps. Set up a clipping rectangle with void al_set_clipping_rectangle(int x, int y, int width, int height), then draw the page bitmap rotated with al_draw_rotated_bitmap(). You may need to use transformations (rotation and translation) to make the final result look good.

Probably the easiest way to achieve an effect like that is to use al_draw_scaled_bitmap(), and scale the bitmap horizontally. (Each page would be an ALLEGRO_BITMAP) It would give you rectangular shaped pages; without perspective. Here's a mock-animation of what it might look like:

Probably the most elaborate way would be to setup a 3D projection and create a mesh of ALLEGRO_VERTEX that deforms like paper as you flip it.

[edit: I don't know why the proportions on that gif are all messed up. hmm. something to do with the forum image embedding?]